Mechanics of Breathing

Pulmonary Ventilation

Pulmonary ventilation is the process by which air is moved into and out of the lungs, enabling gas exchange with the blood. It consists of two main phases:

• Inspiration: Gases flow into the lungs.

• Expiration: Gases exit the lungs.

These phases are essential for maintaining oxygen supply and removing carbon dioxide from the body.

Pressure Relationships in the Thoracic Cavity

Atmospheric Pressure (Patm)

Atmospheric pressure is the pressure exerted by the air surrounding the body. At sea level, it is typically 760 mm Hg, which is defined as 1 atmosphere.

• Respiratory pressures are described relative to Patm:

  • Negative respiratory pressure: Less than Patm

  • Positive respiratory pressure: Greater than Patm

  • Zero respiratory pressure: Equal to Patm

Intrapulmonary Pressure (Ppul)

Intrapulmonary pressure is the pressure within the alveoli of the lungs, also called intra-alveolar pressure. This pressure fluctuates with breathing and always eventually equalizes with atmospheric pressure.

Intrapleural Pressure (Pip)

Intrapleural pressure is the pressure within the pleural cavity. It fluctuates with breathing but is always a negative pressure (less than both Patm and Ppul), usually about 4 mm Hg less than Ppul.

• Fluid level in the pleural cavity must be kept minimal.

• Excess fluid is pumped out by the lymphatic system.

• If fluid accumulates, positive Pip pressure develops, which can cause lung collapse.

Forces Affecting Intrapleural Pressure and Lung Collapse

• Two inward forces promote lung collapse:

  1. Lungs’ natural tendency to recoil: Due to elasticity, lungs always try to assume the smallest size.

  2. Surface tension of alveolar fluid: Surface tension pulls on alveoli to try to reduce alveolar size.

• One outward force tends to enlarge the lungs:

  • Elasticity of chest wall: Pulls the thorax outward.

• Negative Pip is maintained by strong adhesive forces between the parietal and visceral pleurae.

Transpulmonary Pressure

Transpulmonary pressure is the difference between intrapulmonary and intrapleural pressures:

• This pressure keeps lung spaces open and prevents lung collapse.

• The greater the transpulmonary pressure, the larger the lungs will be.

• Lungs will collapse if:

  • or

• Negative Pip must be maintained to keep lungs inflated.

Example: Pneumothorax

Pneumothorax is a condition where air enters the pleural cavity, causing the loss of negative intrapleural pressure and resulting in lung collapse.

Additional info: These notes summarize the introductory concepts of pulmonary ventilation and the pressure relationships that govern lung inflation and collapse, as presented in the referenced textbook slides.